1. Field of the Invention
The present invention is directed to a method for separating a gaseous mixture to produce an enriched gas. More particularly, the present invention is directed to an improved pressure swing adsorption method for separating gaseous mixtures which comprises an adsorption step, a pressure equalization step, a back fill step, and an evacuation and purge step.
2. Description of the Prior Art
Pressure swing adsorption (PSA) is a well known method for separating gaseous mixtures. Pressure swing adsorption involves passing a gaseous mixture at an elevated pressure through a bed of an adsorbent material which selectively adsorbs one or more of the components of the gaseous mixture. Product gas, enriched in the unadsorbed gaseous component(s), is then withdrawn from the bed. The adsorption bed may be regenerated by reducing the pressure of the bed.
The term "gaseous mixture", as used herein, refers to a gaseous mixture, such as air, primarily comprised of two components having different molecular size. The term "enriched gas" refers to a gas comprised of the component(s) of the gaseous mixture relatively unadsorbed after passage of the gaseous mixture through the adsorbent bed. The enriched gas generally must meet a predetermined purity level, for example, from about 90% to about 99.9%, in the unadsorbed component(s). The term "lean gas" refers to a gas exiting from the adsorption bed that fails to meet the predetermined purity level set for the enriched gas.
The selectivity of the adsorbent material may depend on a difference in either adsorption kinetics or adsorption equilibrium. The selectivity of carbon molecular sieves is generally governed by the volume of the pore size and the distribution of that pore size in the adsorbent. Gaseous molecules with a kinetic diameter less than, or equal to, the pore size of the adsorbent are adsorbed and retained in the adsorbent while gaseous molecules with a diameter larger than the pore size of the adsorbent pass through the adsorbent. The adsorbent thus sieves the gaseous molecules according to their molecular size. The adsorbent may also separate molecules according to their different rates of diffusion in the pores of the adsorbent.
Zeolite molecular sieves adsorb gaseous molecules with some dependence upon crystalline size. In general, adsorption into zeolite is fast and equilibrium is reached typically in a few seconds. The sieving action of zeolite is generally dependent upon the difference in the equilibrium adsorption of the different components of the gaseous mixture. When air is separated by a zeolite adsorbent, nitrogen is preferentially adsorbed over oxygen and the pressure swing adsorption method may be employed to produce an oxygen enriched product.
The sieving action of carbon molecular sieves is generally not dependent upon differences in equilibrium adsorption but rather by differences in the rate of adsorption of the different components of the gaseous mixture. When air is separated by carbon molecular sieves, oxygen is preferentially adsorbed over nitrogen and the pressure swing adsorption method may be employed to produce a nitrogen enriched product.
As a gaseous mixture travels through a bed of adsorbent, the adsorbable gaseous components of the mixture enter and fill the pores of the adsorbent. After a period of time, the composition of the gas exiting the bed of adsorbent is essentially the same as the composition entering the bed. This is known as the break-through point. At some time prior to this breakthrough point, the adsorbent bed must be regenerated. Regeneration involves stopping the flow of gaseous mixture through the bed and purging the bed of the adsorbed components generally by venting the bed to the atmosphere.
A pressure swing adsorption system generally employs two adsorbent beds operated on cycles which are sequenced to be out of phase with one another by 180.degree. so that when one bed is in the adsorption step, the other bed is in the regeneration step. The two adsorption beds may be connected in series or in parallel. In a serial arrangement, the gas exiting the outlet end of the first bed enters the inlet end of the second bed. In a parallel arrangement, the gaseous mixture enters the inlet end of all beds comprising the system. Generally, a serial arrangement of beds is preferred for obtaining a high purity gas product and a parallel arrangement of beds is preferred for purifying a large quantity of a gaseous mixture in a short time cycle.
As used herein, the term "adsorption bed" refers either to a single bed or a serial arrangement of two beds. The inlet end of a single bed system is the inlet end of the single bed while the inlet end of the two bed system (arranged in series) is the inlet end of the first bed in the system. The outlet end of a single bed system is the outlet end of the single bed and the outlet end of the two bed system (arranged in series) is the outlet end of the second bed in the system. By using two adsorption beds in parallel in a system and by cycling (alternating) between the adsorption beds, product gas can be obtained continuously.
Between the adsorption step and the regeneration step, the pressure in the two adsorption beds is generally equalized by connecting the inlet ends of the two beds together and the outlet ends of the two beds together. During pressure equalization, the gas within the void spaces of the adsorption bed which has just completed its adsorption step (under high pressure) flows into the adsorption bed which has just completed its regeneration step (under low pressure) because of the pressure differential which exists between the two beds. This pressure equalization step improves the yield of the product gas because the gas within the void spaces of the bed which has just completed its adsorption step has already been enriched.
Gas separation by the pressure swing adsorption method is more fully described in "Gas Separation by Adsorption Processes", Ralph T. Yang, Ed., Chapter 7, "Pressure Swing Adsorption: Principles and Processes" Buttersworth 1987, which reference is incorporate herein by reference.
U.S. Pat. No. 4,376,640, issued to Vo, discloses a pressure swing adsorption method for separating a gaseous mixture which comprises separating a gaseous mixture in a first adsorption bed, pressurizing a second vented and evacuated adsorption bed with lean gas from the first adsorption bed at higher pressure, isolating the second bed to allow the pressurized vessel to decrease in pressure through the adsorption of the gas contained therein, separating the gaseous mixture in a second adsorption bed, and regenerating the first adsorption bed.
United Kingdom patent application no. 2 ,195,097A, to Garrett, discloses a pressure swing adsorption method for separating a gaseous mixture which comprises the improvement of flowing gas from the product reservoir to an adsorption bed whenever the pressure in the product reservoir exceeds that of the adsorption bed.
Japanese patent application no. Sho. 63(1988)-79714, to Hareuma, discloses a pressure swing adsorption method for separating a gaseous mixture which comprises the improvement of cycling three adsorption beds in cycle fashion under very specific pressure swing adsorption conditions.
While the above pressure swing adsorption methods provide improvements in the separation of gaseous mixtures, these methods are not entirely satisfactory. Common problems with pressure swing adsorption methods include low product yield, low product purity, the need for large amounts of adsorbent, and energy inefficient regeneration methods. Hence there is a need for improved pressure swing adsorption methods. The present invention provides such an improved pressure swing adsorption method for the separation of gaseous mixtures in high yield and high purity.